1. Field of the Invention
The present invention relates to a liquid crystal display apparatus for full-color display employing a light source color switching system without using a color filter.
2. Related Background Art
A conventional liquid crystal color display apparatus displays a color image by providing color filters 71 of R (red), G (green), and B (blue) on respective liquid crystal picture elements arranged two-dimensionally as shown in FIG. 11. In the color filter type of liquid crystal panel, respective picture elements correspond to one of R, G, and B, and three picture elements in combination display one color picture element. Therefore, the color filter type of liquid crystal display involves the problem that the resolution becomes 1/3, and the light transmittance becomes 1/3 to cause lowering of the display characteristics in comparison with the monochromatic display having the same number of picture elements.
In one method for solving the problem in the color filter system, signals for R, G, and B are sequentially inputted to a monochromatic display liquid crystal panel, and light source colors are switched over synchronously with the respective color signals. For example, in the system disclosed in Japanese Patent Publication No. 63-41078 (1988), writing the signals and display of R, G, and B are conducted sequentially as shown in FIG. 12.
In conventional monochromatic display liquid crystal panels, the writing and the displaying are conducted for every picture element line by employing a sampling-holding circuit to utilize most of the one vertical scanning period for displaying. In contrast thereto, in the aforementioned light source switching system, the vertical scanning is conducted for each color for the one picture. Therefore, when 3/4 of the maximum one vertical scanning period is allotted to the displaying to obtain brightness, the time for writing is (1/3).times.(1/4)=1/12 for each of the colors. Thus the writing speed is required to be twelve times that of the monochromatic writing. This is not readily realized in view of the performance of TFT.
For color display without increasing the writing speed, one method is to provide a memory for double-speed noninterlace driving in the picture element. In this method, a horizontal scanning line is successively driven, and is transmitted in the vertical retrace period. The period requires several msec depending on the time constant of the horizontal scanning line (gate line) and driving ability of the TFT. More transmission time is required with a larger number of the picture elements.
The memory capacitance should be sufficiently large in comparison with the liquid crystal capacitance. Otherwise, the signal amplitude should be increased. At the capacitance ratio of 10:1, the signal amplitude will be lower by 10%. The liquid crystal should be driven by AC to prevent baking. Therefore, at the maximum amplitude of about 10 V of the AC, the unavailable residual charge in the liquid crystal capacitance is 10% of the signal charge, namely about 1 V, of the memory capacitance. Even if the capacitance ratio is increased to 50:1, the unavailable signal becomes about 200 mV. In the light source switching system in which color signals are sequentially switched, the unavailable signal causes an offensive afterimage to deteriorate the image quality. Furthermore, it is extremely difficult to form the memory capacitance at the capacitance ratio of 50:1 in a limited picture element.
The partitioning of the capacitance into a memory capacitance and a liquid crystal capacitance (including an additional capacitance) causes drop of the signal amplitude. This drop should be compensated by supplying compensating signal voltage from the outside. The memory capacitance and the additional capacitance depend on the film thickness and area of the insulating material. Since the film thickness varies unavoidably in the production process to cause variations of the memory and additional capacitances of the respective picture elements, the liquid crystal capacitance also varies with variation of the thickness of the liquid crystal layer. Therefore, signal voltage from the outside should be adjusted to compensate the variations.
As described above, the high speed writing is indispensable to conventional liquid crystal display apparatuses employing the light source switching system, and accordingly the system for high speed writing involves many technical problems in power consumption, cost, and TFT characteristics, and so forth. Furthermore, the method of providing a memory in the picture element involves problems of picture image deterioration caused by residual charge and undesired increase of the unit picture element area for the memory.
The present invention intends to provide a liquid crystal display apparatus not involving the aforementioned problems. The display apparatus of the present invention realizes full color image display with high fineness and high brightness by employing a liquid crystal display panel without using color filters and without increasing the writing speed, by lengthening the lighting time of the color light source. It is another objective in the display apparatus that variation of the capacitances is decreased.